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-   -   Semroc Midget (http://www.oldrocketforum.com/showthread.php?t=11983)

tbzep 12-21-2012 02:46 PM

Semroc Midget
 
I'm building a Midget and have come across an issue with the kit. It is very possible that I've screwed something up, but I don't see how I could have done it.

The kit is designed to use fishpaper engine blocks mounted flush with the top of the sustainer motor tube, and the other mounted flush with the bottom of the booster tube. I have done this.

When I install two shorty blanks included with the kit, there is nearly an 1/8" gap between the booster and sustainer. I've checked to make sure the engine blocks are flush. The sustainer motor mount is flush with the end of the reducer. The booster motor tube is exposed, so there is nothing that can be wrong with it.

Here's where I need help. Can somebody post the length of both your motor tubes, fishpaper engine blocks, and shorty blanks so that I can see if I've got something with an incorrect length? Also, please measure how far the shorty blank sticks out of the sustainer motor tube assembly. I'd be inclined to think I screwed something up during assembly, but this kit is simple and straightforward and I don't see how I could have done it.

I still plan to finish the kit and fly it. It won't be a problem as I'll just cut 1/8" off one of the motors/blanks to get a proper fit. I fly alone so I won't have any NAR conflicts with RSO's who want to argue I've altered the motor. I see no difference in trimming the end and peeling layers to get them to fit a tight motor mount. Safety Nazis just keep to yourselves. A razor saw isn't going to create enough heat to cause paper to burn and I won't be cutting into propellant.

DaveR 12-21-2012 02:57 PM

Ah, tbzep industries up and running again? :cool:
I figure this one's right up Doug's alley. ;)

Chas Russell 12-21-2012 03:03 PM

I see that Carl is on-line. If you haven't contacted him directly, perhaps he will chime in here if he gets a chance. I have a Midget kit, but have not opened it yet.

Chas

tbzep 12-21-2012 03:59 PM

Quote:
Originally Posted by DaveR
Ah, tbzep industries up and running again? :cool:
I figure this one's right up Doug's alley. ;)


It's only running at partial capacity. A few 1/4A3-0T boosters here, a few 1/2A6-0S boosters there.... :D

Carl@Semroc 12-21-2012 05:36 PM

There must be a short tube that somehow found its way into your kit (or a long engine block.)

The two tubes are:
BT-20AE 1.5"
BT-20M 2.25"
Total 3.75" when stacked.

Two engine blocks at .125" each = .25"
Two empty casings at 1.75" each = 3.50"
Total 3.75" when stacked.

I opened a kit to check the parts and they stack correctly. The overhang of the sustainer engine is .375" into the booster.

tbzep 12-21-2012 07:14 PM

Quote:
Originally Posted by Carl@Semroc
There must be a short tube that somehow found its way into your kit (or a long engine block.)

The two tubes are:
BT-20AE 1.5"
BT-20M 2.25"
Total 3.75" when stacked.

Two engine blocks at .125" each = .25"
Two empty casings at 1.75" each = 3.50"
Total 3.75" when stacked.

I opened a kit to check the parts and they stack correctly. The overhang of the sustainer engine is .375" into the booster.


Thanks Carl. My sustainer overhang is about 1/8" too much. Measuring the difference in total length of the body and the depth to the top of the motor tube gives me a motor tube length of about 1 3/8", so that's the culprit.

It's an easy work-around for me to trim the motor length. I'm comfortable doing that. It looks like my kit was an anomoly, so as long as it doesn't sneak up on somebody else, I'm good to go! :cool:

Doug Sams 12-22-2012 09:07 AM

Quote:
Originally Posted by tbzep
It's an easy work-around for me to trim the motor length. I'm comfortable doing that.
Or just fly it with the gap. I have one I purposely designed that way, to put tape wraps on the ends of the sustainer motors - it's a cluster - acting as the thrust rings. It's not very noticeable.

Another idea, but lots of effort, is to drill/grind out the aft ring in the booster allowing it to slide all the way up. To make up for the loss of the aft ring, you could add an external hook to the booster. I've done that on several of my upscales. It lets me keep ~scale looking tube lengths while accommodating the longer (non-shorty) motors.

Here's one on a Quest T-30 upscale I did. (The fins on the booster somehow ended up with too much rake. I musta been a little careless on the belt sander...:o) Doug .



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